Obtaining uniform saturation of either fat or water using spectrally selective pulses for excitation has be a consistent challenge in clinical magnetic resonance imaging (MRI). Obtaining uniform excitation has also been a consistent challenge. Spectrally selective pulses may excite spins of a particular frequency that is defined by the chemical environment. Spectrally selective pulses used for saturation of a particular chemical species typically excite these spins to around 90 degrees and then spoil this magnetization so that the magnetization does not contribute signal in a subsequent imaging experiment. Other spectrally selective pulses may only excite one particular chemical species for analysis as well.
Chemical shift refers to a variation in the resonance frequency of a nuclear spin due to the chemical environment in the region of the nucleus. The chemical environment in different molecules causes the electron density around the nuclei in a molecule to vary according to the types of nuclei and bonds in the molecule. The chemical shift of a nucleus is the difference between the resonance frequency of the nucleus and a standard, relative to the standard. The chemical shift may be very small and may be reported in ppm. The quantity is given the symbol delta, (d) and is characterized by the following equation:d=(n−nREF)×106/nREF 
The electrons of an atom located in an applied magnetic field circulate about the direction of the applied magnetic field. The circulation of the electrons produces a small magnetic field at the nucleus. The small magnetic field opposes the applied field. Therefore, the magnetic field at the nucleus (B) may be smaller than the applied magnetic field (B0). The difference may be measured by a fraction s.B=Bo(1−s)
Hydrogen in the human body may reside in molecules of fat (—CH2— and/or —CH3—) molecules of water (H2O), and other molecules. The resonance frequency difference between the nuclear magnetic resonance (NMR) signal from these two types of hydrogens is approximately 220 Hz on a 1.5 Tesla imager. The chemical shift difference between these two types of hydrogens is approximately 3.5 ppm. While small, this difference can be exploited in MRI.
MRI chemical shift selective imaging refers to imaging a first set of spins in a volume that have a first chemical shift while excluding from the image other second chemical species that have a different, second chemical shift. Thus, chemical shift selective imaging produces an image from just one chemical shift component in a sample. If the sample being imaged is composed of hydrogens in molecules of water and fat, which are known to have different chemical shifts, then a chemical shift selective imaging technique could acquire an image of either the fat or the water in the object. However, variations in the B0 field may allow some signal from water spins to contaminate a fat spin image or vice versa.
Chemical shift selective imaging may be performed using a saturation method. In the saturation method a frequency selective saturation pulse is applied before the standard imaging pulses of a sequence. The saturation pulse is intended to zero out the magnetization from the component to be suppressed. When the standard imaging sequence follows it is not supposed to detect signal from the suppressed component. However, a consistent challenge in clinical MRI has been obtaining uniform saturation of a component to be suppressed using spectrally selective pulses.
Spectrally selective pulses may be employed to excite only the hydrogen atoms in fat molecules. The spectrally selective pulses are designed not to affect hydrogen atoms in other molecules (e.g., water). Therefore, the water molecules should contribute to a final image while the fat molecules should not. Images acquired in association with these spectrally selective pulses are referred to as “fat suppressed” or “fat saturated” images. These images may also be described as having a “saturation region” or “saturation band.” Fat suppressed images are clinically important where a physician needs to “see through” the fat to see underlying items. If the image could not see through the fat, then fat could obstruct underlying pathology, and so on. While spectrally selective pulses that produce fat suppressed images are described, one skilled in the art will appreciate that example systems and methods may apply to other spectrally selective pulses and other chemical shifts.
Spectrally selective pulses may be compromised by variations in the main magnetic field (B0). While a significant amount of B0 inhomogeneity can be dealt with through shimming, variations introduced by, for example, magnetic susceptibilities in a patient may limit the ability to correct for inhomogeneity through shimming.
The magnitude of the chemical shift may be measured in parts per million. A fat saturation pulse may have a narrow bandwidth to facilitate exciting only fat spins. However, inhomogeneities on the order of a few parts per million across a patient after shimming may result in “spectral broadening”. Spectral broadening may result in some fat spins lying outside the bandwidth of the saturation pulse. Spectral broadening disrupts the ability to effectively and uniformly perform saturation excitations throughout an entire sample. Since the entire sample cannot be saturated, some spins that were not supposed to contribute signal to an image may end up contributing signal to the image.
Some conventional attempts to address spectral broadening have focused on shimming. The conventional attempts may have included shimming hardware and software. Since shimming improvements have been sought for years, reductions in spectral broadening through improved shimming may only provide incremental or inconsequential results. Dynamic shimming, where shims may be tailored slice-by-slice, may provide additional improvements in the future.
Other conventional attempts to address spectral broadening have included performing multiple acquisitions at different echo times and then performing offline reconstruction to separate different spectral components. Performing multiple acquisitions may unacceptably lengthen acquisition time.
Still other conventional attempts to address spectral broadening have included using multi-dimensional pulses that utilize gradients during excitation to address frequency differences in space. However, the improvements sought by multi-dimensional pulses may be limited by gradient performance. To the extent that improvements are possible, the improvements may require optimization routines. While improvements may be available, multi-dimensional pulses may unacceptably lengthen acquisition time. Additionally, multi-dimensional pulses may impact specific absorption rate (SAR).
While chemical shifts and fat saturation have been described above, other challenges with uniformity have been faced over the years in MRI. For example, uniform excitation in a region, and uniform selective excitation in a region have been a challenge. Additionally, uniform selective excitation in a first region with simultaneous signal cancelling from a neighboring region has been a challenge.